1. Field of the Invention
The present invention relates to an optical probe that is used to produce optical tomographic images by OCT (Optical Coherence Tomography) measurement. Further, the present invention relates to a tomographic image production apparatus using the optical probe.
2. Description of the Related Art
Conventionally, when an optical tomographic image of a tissue of an organism (living body) is obtained, an optical tomographic image obtainment apparatus using OCT measurement has been used in some cases. The optical tomographic image obtainment apparatus has been used mainly to examine the fundus of eyeball (eyeground), the anterior segment of eyeball, and skin. Further, the optical tomographic image obtainment apparatus has been used in examination of various other regions of the body, such as observation of the walls of arteries using a fiber probe, and observation of digestive organs by inserting a fiber probe through a forceps channel of an endoscope. In the optical tomographic image obtainment apparatus, low coherent light output from a light source is divided into measurement light and reference light. Further, reflection light that is reflected from an object to be measured (measurement target) or backscattered light therefrom when the measurement target is irradiated with the measurement light is combined with the reference light. Then, an optical tomographic image is obtained based on the intensity of interference light of the reflection light and the reference light.
Basically, there are two types of OCT measurement, namely TD-OCT (Time Domain OCT) measurement and FD-OCT (Fourier Domain OCT) measurement. In the FD-OCT measurement, the intensity of interference light is measured for each spectral component of light without changing the optical path length of the reference light and that of the measurement light. Further, spectral analysis, such as Fourier transformation, is performed on the obtained spectral interference intensity signals by using a computer, thereby obtaining the distribution of the intensity of reflection light corresponding to the depth position of the measurement target.
When a tomographic image is obtained by the OCT measurement, it is necessary to irradiate the measurement target with measurement light in such a manner to scan the measurement target. Meanwhile, as MEMS (Micro Electro Mechanical Systems) techniques have developed in recent years, leading-end (tip) optical systems are attached to MEMS motors provided within the outer tubes of probes. Further, optical probes in which the leading-end optical systems thereof are rotationally moved by the MEMS motors with respect to axes have been disclosed (for example, please refer to Jianping Su et al., “In Vivo Three-Dimensional Microelectromechanical Endoscopic Swept Source Optical Coherence Tomography”, Optics Express, vol. 15, No. 16, pp. 10390-10396, 2007). However, since it is necessary to install the MEMS motors within the probes, it is difficult to reduce the diameters of the probes. Especially, when optical probes for OCT are inserted into the body cavities of patients through forceps holes (openings) of endoscopes, since the diameters of the forceps holes are mainly 2.6 mm or 1.8 mm, it is desirable that the diameters of the optical probes are less than or equal to 1.6 mm.
Meanwhile, in the OCT measurement, a rotary joint is generally used to rotationally scan the measurement target with the measurement light (for example, please refer to Japanese Patent No. 3104984). An optical probe for OCT disclosed in Japanese Patent No. 3104984 includes a sheath that is inserted into the inside of a subject to be examined (examination subject), and a flexible shaft that extends in the longitudinal direction of the sheath within the sheath, and that can rotate with respect to the axis of the shaft. Further, the optical probe for OCT includes an optical fiber coated with the flexible shaft and a leading-end optical system that deflects light output from the optical fiber substantially at a right angle with respect to the longitudinal direction of the optical fiber. Further, the flexible shaft is rotated through a gear by a motor arranged at the base end of the optical probe, thereby rotationally moving the leading-end optical system with respect to the axis.
However, when the leading-end optical system attached to the leading end (tip) of the optical fiber is moved to rotationally scan the target by rotating the base portion of the optical fiber, as disclosed in Japanese Patent No. 3104984, there is a problem that the rotation of the leading-end optical system becomes irregular because of friction between the shaft and the probe outer-tube (outer-cylinder or sheath) or the like. Specifically, while the leading-end optical system makes one turn, the rotation speed of the leading-end optical system becomes low in a certain scan area, and becomes high in another scan area or the like. Consequently, scan lines with the measurement light become dense in a certain scan area, and the scan lines with the measurement light become thin in another scan area. Meanwhile, in processing for producing tomographic images, the tomographic images are produced by arranging, at equal intervals, a predetermined number of scan lines for each rotation of scan. Therefore, there are cases in which a position of the measurement target that is actually irradiated with measurement light differs from a position of the measurement target that is represented in the tomographic image, thereby deteriorating the image quality of the tomographic image.